1. Field of the Invention
This invention relates to a method of producing thin steel sheets having an improved processability.
2. Related Art Statement
Thin steel sheets having a thickness of not more than 6 mm and used for building materials, vehicle bodies for automobile, can materials, various surface treating black plates and so on are roughly divided into hot rolled steel sheet and cold rolled steel sheet.
Heretofore, these thin steel sheets are produced by the following general production steps:
At first, a steel slab of about 200 mm in thickness is produced by a continuous casting process or an ingot making-blooming process, and then hot rolled to a thickness of 2-6 mm. That is, the slab is subjected to a rough hot rolling to form a sheet bar of about 30 mm in thickness and further to a finish hot rolling to form a hot rolled steel sheet of 2-6 mm in thickness.
Furthermore, the hot rolled steel sheet is pickled, cold rolled to a given thickness, and subjected to a recrystallization treatment by box annealing or continuous annealing to form a cold rolled steel sheet.
In the above-mentioned conventional procedure for the production of the thin steel sheet, it is a greatest drawback that the production installation is enormous. Particularly, in the hot rolling step required for the production of hot rolled steel sheet and cold rolled steel sheet, 4-5 rough rolling machines are usually used at rough rolling stage and a tandem type rolling machine usually composed of 6-7 stands is used at finish hot rolling stage, so that the installation becomes necessarily enormous.
On the other hand, in the production of cold rolled steel sheets, the production step is a serious drawback to be lengthened. That is, the energy, labor and time required for obtaining the product become enormous, and also various problems are unfavorably caused in the quality of the product, particularly surface properties thereof at such a long step.
There have hitherto been proposed some technics on the production of the thin steel sheet. For instance, Japanese Patent laid open No. 47-30,809 discloses a hot rolling apparatus wherein a cooling device, a rolling roll and a coiler are arranged behind the usual finish hot rolling stand. According to this apparatus, the hot rolled steel sheet is continuously subjected to a low temperature hot rolling after the coiling, whereby a drawability equal to that of the cold rolled steel sheet is obtained even when omitting the cold rolling step. However, compact equipment and high productivity are not satisfied due to the use of the usual finish hot rolling stand.
In Japanese Patent laid open No. 59-101,205 is disclosed an apparatus for producing a hot rolled steel sheet from a continuously cast steel material wherein a shear, a coiling device, a heating furnace, a decoiling device, a coiling furnace, a rolling machine and a coiling device are arranged behind the continuous casting machine. According to this apparatus, however, the properties based on the omission of cold rolling step and annealing step are not attained because only the hot rolled steel sheet is produced.
In Japanese Patent laid open No. 59-229,413, there is disclosed an apparatus for producing extra-fine ferrite steel wherein a rolling machine incorporated into a common housing and composed of an entry side roll set having a work roll diameter of 600 mm and an exit side roll set having a work roll diameter of 200 mm at a distance between the work rolls of 2,000 mm is arranged behind a continuous casting machine of small sectional area, and a salt bath and a coiler are arranged therebehind. However, only the hot rolled steel sheet is produced by this apparatus, so that it is impossible to omit the cold rolling step and cold rolling-annealing step. Further, it is difficult to obtain a uniform texture in the steel sheet.
In Japanese Patent laid open No. 60-108,101 is disclosed an apparatus for producing thin metal plates wherein a temperature adjusting device, a coil box, a hot rolling device and a coiler are arranged behind a continuous casting machine. However, only the hot rolled steel sheet is produced even by this apparatus, so that it is impossible to omit the cold rolling step and cold rolling-annealing step.
All of the aforementioned conventional technics are excellent as an apparatus for producing thin steel sheets, but do not satisfy all of the conditions such as omission of cold rolling step and cold rolling-annealing step, high productivity and compact equipment.